Apparatus and method for detecting the presence of gas in a borehole flow stream

ABSTRACT

A borehole apparatus detects the presence of gas within a formation fluid sample. A light source transmits light rays to an interface between the fluid sample and the flow line. The interface reflects the light rays toward a detector array. The detector array detects light rays having angles of incidences extending from less than the Brewster angle to more than the critical angle for gas. A data base stores information concerning the Brewster angle and critical angle of gas for a plurality of gas volume categories. A processor determines the percentage of gas present in the formation fluid sample and categorizes the fluid sample as high as, medium gas, low gas, and no gas based on the signal from the detector array and the information from the data base.

This is a divisional application of copending U.S. Ser. No. 07/574,filed Aug. 28, 1990 for "Apparatus and Method for detecting the Presenceof Gas in a Borehole Flow Stream".

The invention concerns an apparatus and a method for analyzing thecomposition of multiphase formation fluids and more specificallyconcerns an optical apparatus and method for detecting the presence ofgas in a flow stream that comprises oil, water, gas or particulateswithin a borehole.

FIELD OF THE INVENTION

U.S. Pat. No. 4,860,581 to Zimmerman et al. ("the Zimmerman patent")concerns a downhole tool for the detection of formation properties. TheZimmerman patent is assigned to the same assignee as this invention. Thetool of the Zimmerman patent measures pressure in the borehole duringdrawdown and buildup tests to estimate permeability based on anassumption that flowing fluid is incompressible. However, this is notthe case. The pressure drop is actually quite large, a few thousand psibeing typical. Many formation crude oils, particularly those under a gascap, will evolve gas under such a pressure drop. In fact, individualpressure buildup curves are inconsistent when incompressible fluid flowis assumed.

Differential flow velocities between gas and liquid phases can be largedue to their very different viscosity and density. Thus, when gasevolves in formation fluids, samples of formation fluid may notaccurately represent the composition of the formation fluid. When gasevolves, the resulting sample can be enriched in light hydrocarbonscompared to formation crude oil. Gas bubbles can block pores, reducingpermeability and gas that evolves in the formation can reduce gas drive.

Japanese patent No. 61-11636, entitled Liquid Discriminating Sensor,describes a refractometer which determines the index of refraction of asubstance by determining the critical angle. Significant differencesexist between the Gas Detector of this invention and the LiquidDiscriminating Sensor in both purpose and design. The purpose of theLiquid Discriminating Sensor is to determine the identity of the onlycomponent of a liquid sample. The Liquid Discriminating Sensor does notwork for multiphase samples. The Liquid Discriminating Sensor measuresthe unknown index of refraction of a liquid sample with high precisionin order to distinguish between the similar indices of refraction ofdifferent liquids, thereby determining sample identity. The cost of thehigh precision is that only a limited angular range can be covered.

Thus, the detection of gas phase and the categorizing of percent gasvolumes of the formation fluid would improve permeability estimates,sampling accuracy, and production pressure estimates. The downholedetection of gas in the flow stream would allow a more accuratedetermination of bubble point using pressure gauges and thedetermination of the minimum pressure at which the formation can beproduced.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide an apparatus fordetermining the presence of gas within formation fluid that may includewater, gas, oils and particles.

It is another object of the invention to provide a downhole apparatusfor determining in situ the presence of gas within the formation fluid.

It is another object of the invention to provide an apparatus that useslight for detecting the presence of gas in a formation fluid.

In one embodiment, the invention comprises a borehole apparatus and amethod for categorizing volumes of gas in a multiphase fluid obtainedfrom a formation. The fluid comprises at least one of oil, water, andgas. The apparatus preferably comprises a flow line for containing thefluid and a light source for transmitting light toward the fluid in theflow line. A prism transmits light from the source to the fluid andforms an interface with the flow line. The interface reflects light fromthe source and a detector array detects the light. A data base storesinformation relating to a plurality of categories of determined gasvolumes of formation fluid. A processor obtains data concerning thereflected light from the detector array and the information from thedata base to determine a category of gas volume of the fluid in the flowline. The purpose of the Gas Detector is to detect the presence of gasin a multiphase flow stream. The flow may contain gas, immiscibleliquids, such as oil and water, and solids, such as sand and clay. Inaddition, the flow stream can be quite erosive. The Gas Detector mustalso work under conditions of very high pressure, up to 20,000 psi. Dueto the high pressures, the gas density can become appreciable;correspondingly the gas index can become large (˜1.2). Knowledge of thepressure and temperature provides the index of refraction of the(natural) gas. The Gas Detector detects the presence of gas of knownindex of refraction. Thus the Gas Detector does not measure the index ofrefraction of the multiphase flow stream to determine the flow streamconstituents. As such, the gas detector is not a refractometer.

In addition to the different purposes of the Gas Detector and the LiquidDiscriminating Sensor, there are significant design differences. The GasDetector must cover a wide angular range; thus a light source of verylarge numerical aperture is needed (˜0.6). Constraints on the opticaldesign are also imposed by the high pressures attainable in the flowstream, which must be supported by the optical window. In the opticaldesign of the Liquid Discriminating Sensor, the window depicted wouldnot be able to withstand a high pressure, erosive flow stream like thatin a borehole.

In addition, the large angular range necessitated for high and variablepressure gas detection would be very difficult to achieve with theoptical design of the Liquid Discriminating Sensor. The Gas Detectoremploys a source of numerical aperture (NA) of ˜0.6; a lens providingthis NA would have an F number of 0.8 for collimated light source. Toachieve the same NA and image distance with the point source of theLiquid Discriminating Sensor, the lens must be even faster. Lenses thatare this fast are very difficult to work with and are frequentlyineffective in achieving large NA due to reflection losses for themarginal light rays. In addition, the retaining force that must beapplied to the sapphire window places significant limitations on thedesign and location of the lens. Finally, when sampling multiphase flowstreams, inaccurate results are easily obtained if a single point of theflow stream is sampled in the manner of the Liquid DiscriminatingSensor. The Gas Detector samples an entire area of the flowstream-sapphire interface, thus making the Gas Detector much lesssusceptible to inaccuracies resulting from droplets or films on a windowat that interface. The Gas Detector is a unique device which satisfies arecently developed niche in the oil field.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic diagram of a borehole logging tool having a gasdetector module for analyzing the composition of a formation fluid.

FIG. 2 is a perspective cutaway view of the gas detector module of FIG.1.

FIG. 3 is a schematic diagram of the gas detector module of FIGS. 1 and2.

FIG. 4 illustrates output signals of an array of photodetectors in thegas detector module of FIGS. 2 and 3.

FIG. 5 illustrates a plot of reflected intensity verses angle ofincidence of light incident on an interface between a sapphire prism andwater, and gas.

FIG. 6 illustrates a plot of gas detector signal verses gas volumefraction for flow streams containing crude oil and gas.

FIG. 7 illustrates a flow chart of steps that the gas detector of FIGS.1-3 follows in categorizing the volume of gas in the flow stream.

FIG. 8 illustrates a plot of gas detector signal verses low gas volumefraction and emulsion flow streams.

FIG. 9 illustrates the effect of a liquid film between the sapphireprism and the flow stream of FIG. 2.

DETAILED DESCRIPTION

The invention is applicable to both production logging and boreholeinvestigative logging.

FIG. 1 is a schematic diagram of a borehole logging tool 10 having a gasdetector module for analyzing the composition of a fluid from aformation 14. The borehole logging tool 10 hangs from the lower end of amulticonductor cable 15 in a borehole 12. A winch 13 on the Earth'ssurface spools the multiconductor cable 15 in the usual manner. On thesurface, the multiconductor cable 15 electronically connects to aprocessing system 18. The processing system 18 receives data over themulticonductor cable 15 and analyzes the data to determine gasconcentrations, as described below.

The logging tool 10 has a elongated body 19. The body 19 contains anassembly 20 that extends from the body 19 for admitting fluid into thebody 19. The body 19 also contains another assembly 21 that extends fromthe body 19 for anchoring the body 19 in the borehole 12. These twoassemblies 20 and 21 preferably extend from opposite sides of the body19. The assembly 20 for admitting fluid seals off or isolates selectedportions of the wall of the borehole 12 to establish pressure or fluidcommunication with an adjacent earth formation. U.S. Pat. Nos. 4,396,254and 3,859,851 describe suitable assemblies for obtaining fluids and areassigned to the assignee of this invention. The disclosures of thosepatents are incorporated by reference.

Fluid admitted by the assembly 20 flows through a module 25 of thelogging tool 10 that analyzes the composition of the formation fluid.U.S. patent application Ser. No. 07/418,748 describes a suitableembodiment of such a module and is assigned to the same assignee as thisinvention. The disclosure of that application is incorporated byreference. The fluid also flows through a gas detector module of thelogging tool 10. The fluid then exits the logging tool 10 through a port(not shown) or travels to one or more collecting chambers that receiveand retain fluids obtained from the formation 14. The processing system18 and a downhole system 16 control the assembly for admitting fluids,the assembly for anchoring the body, the analyzer module, gas detectormodule, and the flow path to the collecting chambers.

FIG. 2 is a cut-away perspective view of a gas detector module 26according to this invention. The gas detector module 26 comprises a highpressure cell 29 of stainless steel that encloses the high pressure flowline and other internal components of the gas detector module 26 withinthe borehole environment, for example. One internal component of the gasdetector module 26 is a flow line, such as 30 of FIG. 3, that containsthe formation fluid in the module for analysis of the fluid. The flowline 30 is considered to contain any fluid that is held by or flowsthrough the flow line 30. The flow line 30 has a window that comprisesan optical element such as a prism 34. A surface of the prism 34contacts the formation fluid in the flow line 30, creating an opticalinterface 34a between the prism 34 and the fluid. The interface 34arefracts light into the fluid and reflects light in an optical system asdescribed below concerning FIG. 3. Preferably, the prism 34 is sapphire,which is transparent to near-infrared and visible light and is resistantto abrasion by the formation fluid.

FIG. 3 is a schematic diagram of an optical system comprising the gasdetector module 26 of FIG. 2 according to this invention. The inventorshave found that by arranging a prism and detectors to monitor reflectedintensity of light in an angular range that extends from less than theBrewster angle to more than the critical angle for gas, the presence ofgas in the formation fluid is revealed, as described below.

The optical system includes a light source 31 that generates lighttoward formation fluid contained in the flow line 30. The light source31 comprises an infrared emitting diode or a light emitting diode, forexample. The light generated by the light source 31 has sufficientangular width for compatibility with other components of the opticalsystem, which are described below.

The light source 31 transmits light to a p-polarizing filter 32.P-polarized light is preferred over s-polarized or unpolarized lightbecause p-polarized light enhances contrast of light that passes througha cylindrical lens 33 and reflects from the interface 34a in an angularrange between a Brewster angle and a critical angle. A Brewster angle isthe angle of incidence of light that reflects from a surface at whichthe reflectivity for light having an electrical vector in the plane ofincidence becomes zero. The critical angle is the smallest angle ofincidence at which total internal reflection from an interface occurs.

The cylindrical lens 33 forces the light only on the interface 34abetween the prism 34 and the formation fluid, and not on the sides ofthe cell 29 nor any part of the prism 34 that contacts the body 19 ofthe cell 29. Focusing the light increases the intensity of the lightthat is transmitted to and reflected from the interface 34a between theformation fluid and the prism 34. Light, such as rays A, B, and C, thenenters the prism 34 and strikes the interface 34a at a desired range ofangles of incidence that extends from less than the Brewster angle togreater than the critical angle of gas in the flow stream in thevicinity of the interface 34a. The related light rays are thentransmitted through the opposite face of the prism 34 to an array ofdetectors 35.

The array of detectors comprises a linear array of eight photodetectors1-8 that are sensitive to infrared light, in one embodiment. Eachdetector 1-8 of the array 35 is positioned to receive light that isreflected by the interface 34a at a corresponding angle of reflectance.Accordingly, the array 35 of detectors is positioned such that eachdetector 1-8 is illuminated by light rays of a particular angle ofreflection and, thus, of a particular angle of incidence. In anotherembodiment, one detector can be used instead of an array of detectors todetect light between the Brewster angle and the critical angle. Also, apolarizing filter can be placed at the single detector or array ofdetectors, instead of between the light source 31 and the cylindricallens 33.

For example, light rays, such as A, that intersect the interface 34anearest the light source 31 have relatively small angles of incidence.Accordingly, a substantial amount of light ray A enters the flow streamand the interface 34a reflects a small amount AA of the ray toward thedetector 1. Light rays, such as C, that intersect the interface 34afarther from the light source 31 have larger angles of incidence and areentirely reflected by the interface 34a. None of light ray C enters theflow stream in the flow line 30 because the interface 34a reflects allof the ray toward another detector 8, assuming the flow line is filledwith a gas. The interface 34a reflects all of the light ray C becausethis ray is at the critical angle of gas, for instance.

FIG. 4 plots a curve 42 that represents the output signal for eachdetector 1-8 in an array of detectors. The output signal for eachdetector 1-8 indicates the intensity of the light received by eachcorresponding detector of FIG. 3. For example, the low intensity lightray AA that the interface 34a reflects towards detector 1 appears as alow amplitude signal 41 in the plot of FIG. 4 for detector 1. The fullintensity light ray CC that the interface 34a reflects towards detector6 appears as the peak signal 46 in the plot of FIG. 4 for detector 6.This peak signal 46 represents the critical angle of gas of the flowstream in the flow line 30. The curve 42 of FIG. 4 also indicates theBrewster effect at 44 where the signal falls to zero. This zero signalis the output of detector 5.

FIG. 5 illustrates the relationship between reflected light intensityand the angle of incidence for light that is incident on an interface,such as 34a, between a sapphire lens and water, shown as a dashed curve46, and gas, shown as a solid curve 48. In the gas curve 48, a sharptransition 50 occurs from a zero signal at approximately 30 degreesincidence, the Brewster angle, toward the critical angle. Thistransition 50 in the curve 48 of FIG. 5 corresponds to the zero signal44 in the curve 42 of FIG. 4. At 30 degrees incidence the water curve 46has a broad minimum and little variation in intensity. An oil curvewould have a similar broad minimum.

When only water or oil is present in the formation fluid, the detectorsof the array 35 would indicate the presence of little if any light,because water and oil would reflect light at a great angle of incidence.Such an angle of incidence would be outside the range extending from theBrewster angle to the critical angle for gas, and to which the detectors1-8 are responsive. For example, the water curve 46 of FIG. 5 indicatesthat water in a flow line would reflect light at greater than 45 degreesincidence, which is outside such a range.

As described below concerning FIGS. 6-8, the processing system 18receives and examines the output of the detector array 35 and correlatesa peak intensity of reflected light to the critical angle for gas and azero intensity to the Brewster angle of gas. The processing system 18then examines the slope and other characteristics of the resulting curvebetween these angles to categorize gas volumes of the multiphase fluidformation in the flow line 30.

Preferably, a computer program executes steps to examine the output ofthe detector array 35 and the resulting curves. However, amicroprocessor or a microcomputer could also perform these steps. Theprocessing system 18 preferably includes a data base that storesinformation that corresponds to the data represented by the FIGS. 4, 5,7, and 8, for example. The processing system compares the output of thedetector array 35 to the information of the data base to label the fluidin the flow line as one of four gas volume categories. The data basecould comprise a look-up table in an EPROM or data values in a computerprogram, for example. The data base can be located on the surface withthe processor or as a separate element downhole with the logging tool10.

The effects of flow stream geometry must be considered for optimum useof this invention. For equal gas volume fractions of fluid flow, the gasdetector responds differently to slug flow streams compared to bubble orfoam flow streams. For slug flow streams, the detector sees 100% gaspassing through the flow line 30, then 100% water, for example. However,the average of a signal from the gas detector is linear in relation togas volume fraction.

For bubble or foam flow streams, the gas detector sees some amount ofliquid and gas bubbles at a variety of depths of the flow stream.Bubbles passing the prism 34 reflect light toward the detectors 1-8differently than when no bubbles are present at the flat interface 34abetween the prism 34 and gas in the flow line 30. These differentreflections and bubble depths cause the output signal from the detectors1-8 to deviate from the signal that would occur for a slug flow streamhaving the same gas volume fraction. In addition, the contents of thegas detector cell 29 also affect the output signal of the gas detectormodule 26. For example, due to the different slip velocities of gas,liquids and solids over the prism surface of the cell 29, thecomposition of cuts of the formation fluid can differ from the actualcontents of the cell 29. For this reason, results produced by the gasdetector module 26 must be viewed as semiquantitative instead ofquantitative, and a process for analyzing resulting data is required.

FIG. 6 illustrates gas volume fractions of 90% and 70% of multiphasefluids with curves 50 and 52, respectively. Curves 50 and 52 indicatelarge gas step heights of approximately 0.91 and 0.62, respectively.FIG. 6 also illustrates gas volume fractions of 50%, 30% and 3% withcurves 54, 56, and 58, respectively. Curves 54, 56, and 58 indicate gasstep heights of approximately 0.20, 0.08, and 0.01, respectively. Thegas step heights of these curves are used to distinguish between gasvolume categories of a multiphase fluid flow, as discussed below.

FIG. 7 is a flow chart illustrating steps for analyzing output signalsfrom the detector array 35. A variety of flow stream geometries and gasvolume fractions were studied by the inventors to develop the steps ofthis process. Application of the steps to output signals of the detectorarray and to information in the data base of the processing system 18 isused to categorize formation fluid into four categories of gas volumefraction: high gas, medium gas, low gas, and no gas.

To determine high gas and medium gas volume categories, it is necessaryto consider only gas step height ΔS₈₁. Gas step height ΔS₈₁ =S8-S1,where S8 is the normalized voltage of the output signal produced by adetector corresponding to a large angle of incidence (in this casedetector 8), and S1 is the normalized voltage of the output signalproduced by a detector corresponding to a small angle of incidence (inthis case detector 1). The gas step height increases monotonically withgas volume fraction and is, therefore, most noticeable at the high gasand medium gas volume categories.

If gas step height ΔS₈₁ is larger than 0.3 (where 1 is full scale), asin block 60 of FIG. 7, the associated flow stream is labeled as high gascontent, as indicated by block 60a. This label mostly represents flowstreams with gas volume fractions larger than 70%. If ΔS₈₁ is less thanor equal to 0.3 and greater than 0.04, as shown in block 62, theassociated flow stream is labeled as a medium gas content, as indicatedby block 62a. This label mostly represents flow streams with gas volumefractions between 11% and 60%. If the gas step height is less than orequal to 0.04, as shown in block 63, further analysis is required todistinguish between low gas and no gas volume categories.

To fully distinguish between low gas and no gas categories, it isnecessary to consider both baseline shift S₁ and curvature Cu, alongwith the gas step height ΔS₈₁. Baseline shift S1 is the normalizedvoltage of the output signal received from detector 1. Curvature Cu isroughly the ratio of the slope of the curve between the Brewster angleand the critical angle, in this case the difference between the outputsignals of detectors 5 and 3 as indicated by FIG. 6, and the slope ofthe curve prior to the Brewster angle, in this case the differencebetween the output signals of detectors 3 and 1, for example.

The inventors have studied a large number of flow streams to develop anempirical means of distinguishing between flow streams containing lowgas volume fractions and multiphase flow streams containing very littleor no gas, such as emulsions. The different curvatures and baselines ofcurves representing such flow streams are used to distinguish betweenlow gas and no gas categories. Emulsion streams containing no gasexhibit a curvature Cu similar to flow streams of low gas fraction, buthave a larger baseline shift. Flow streams of emulsions containing smallamounts of gas have a baseline shift similar to emulsions containing nogas but exhibit larger curvatures Cu.

FIG. 8 illustrates normalized voltages of detector output signals forlow gas fraction and emulsions flow streams. Curve 72 and curve 76respectively represent flow streams of 100% oil and flow streams of 100%emulsion. Curve 76 indicates that a flow stream of 100% emulsion has abaseline shift S1 of approximately 0.036. Curve 72 indicates that a flowstream of 100% oil has a baseline shift of approximately 0.001.

Thus, if ΔS₈₁ is less than or equal to 0.04 and greater than 0.01, as inblock 63 of FIG. 7, and the baseline shift S1 is small S1<0.004, as inblock 64 of FIG. 7, the associated flow stream is labeled as low gascontent, as indicated by block 64a. If the baseline shift S1 is notsmall S₁ >0.004 and the curvature is large Cu>2, as in block 66 of FIG.7 the associated flow stream is also labelled as having low gas content,as indicated by block 66a. This label mostly represents flow streamswith gas volume fractions between 1% and 10%. However, if the baselineshift is not small S1>0.004 and the curvature is small Cu<2, theassociated flow stream is labeled as having no gas content, as indicatedby block 68a. Finally if ΔS₈₁ is less than or equal to 0.01, as in block70 of FIG. 5, the associated flow stream is labeled as having no gascontent, as indicated by block 70a. This label represents flow streamshaving a gas volume fraction of 0%.

Because the steps illustrated by FIG. 7 have been developed empiricallyfrom data using a specific gas detector geometry, the exact values ofgas step height ΔS₈₁, baseline shift S₁, and curvature Cu whichdetermine the gas amount categories may change. Nevertheless, the basicconcept of using the gas step height, baseline shift, curvature todetermine gas volume categories is still applicable for differenthardware configurations.

FIG. 9 illustrates the effects of a liquid film between the sapphireprism and the gas phase of FIG. 3. Basically, a liquid film between thesapphire prism and the gas phase displaces light rays a distance D_(r),yet has no effect on the results obtained by the technique of thisinvention, as described below.

An interface 80 between the gas phase of the flow stream and the liquidfilm is assumed to be parallel to an interface 82 between the film andthe sapphire prism 34. For a given angle of incidence, Snell's law canbe used to relate the indices of refraction and angular variables forthese dielectrics:

    n.sub.s sin(θ.sub.s)=n.sub.f sin(θ.sub.f)=n.sub.g sin(θ.sub.g)

where the subscript s refers to the sapphire prism, the subscript frefers to the film having a thickness t, the subscript g refers to thegas phase of the flow stream, θ_(s), θ_(f), and θ_(g) are angles shownin FIG. 9, and correspond to angles of incidence of light rays. Thisequation shows that the film has no effect on the Snell's law relatingangles and indices of refraction for the prism and gas alone. Thus, thecritical angle for the angle of incidence in the prism is still sin⁻¹(ng/n_(s)). However, the critical reflection does not occur at theinterface 82 between the prism and the film, but at the interface 80between the film and the gas phase. Consequently, the criticallyreflected light ray is displaced in the prism compared to the case inwhich no film is present between the prism and the gas phase. Thedisplacement d_(r) of the light ray in the film is: ##EQU1##

Though the light ray has been displaced a distance d_(r), the resultsobtained by the invention are not affected, because this displacement isvery slight (film thickness which is generally less than one detectorwidth) and because this invention is not concerned with which detectorsare illuminated by a ray of light, but the gas step increase that occursbetween illuminated detectors. In the case of no film, the inventiondetermines the gas step height that occurs between the Brewster angle,and the critical angle of gas, regardless of which particular detectorssense these angles. In the case of a thick film between the gas phaseand the prism, the light ray would be displaced a distance d_(r) suchthat the Brewster angle and the critical angle would be sensed bydifferent detectors than those indicated in FIG. 4. However, the samegas step height would occur, which would be processed according to thetechnique of FIG. 7 and would, therefore, indicate the same percent gasvolume as in the case of no film. For example, in the case of a thinfilm between the gas phase and the prism, where n_(s) is 1.75, n_(f) is1.4 and θ_(s) (the critical angle) is 34.9 degrees, d_(r) is equal to1.68t. The thickness t of such a film is less than 200μ and the distanced_(r) in this example is less than 336μ. Thus, the inventionsuccessfully detects the percent gas volume of a high pressure gas in aflow stream despite the presence of a liquid film on the prism for casesin which the index of refraction of borehole high pressure gases isdifferent than the index of refraction for borehole liquids.

The invention concerns an apparatus and method for analyzing thecomposition of a fluid comprised of one or more of water, oil, and gas.However, while particular arrangements with light sources and opticalelements are shown, it is evident that other light sources, such asultraviolet or visible light sources, as well as other elements forguiding the light to and from the sample, such as lenses, may beutilized. Likewise, other elements for measuring the intensity of thereflected light rays could be utilized. Other gas volume fractions canbe chosen to define the gas volume categories. If only one detectorelement is used, then the angular range would essentially be centered atthe gas critical angle and would not include the Brewster angle. Nolight would be obtained from the gas Brewster angle (which is near theliquid Brewster angle) so no information is gained by including theBrewster angle. For multichannel operation the Brewster angle isincluded (desirable, but not necessary) to produce contrast. Further,while the invention was described as having data processing means on thesurface of the formation 14, it will be appreciated that such processingmeans could be at least partially located downhole. Therefore, it willbe apparent to those skilled in the art that other changes andmodifications may be made to the invention as described in thespecification without departing from the spirit and scope of theinvention as so claimed.

I claim:
 1. A borehole apparatus for categorizing volumes of gas in amultiphase fluid obtained from a formation, the fluid comprising atleast one of oil, water, and gas, the apparatus comprising:containingmeans for containing the fluid; source means for transmitting lighttoward the fluid in the containing means; optical means for transmittinglight from the source means to the fluid and having an interface withthe containing means, the interface comprising a means for reflectinglight from the source means at an angle of incidence within a range thatextends from less than a Brewster angle to more than a critical angle ofgas; detector means for detecting light which is reflected from theinterface; storing means for storing information related to a pluralityof categories of determined gas volumes of formation fluids; andprocessing means for obtaining data concerning the angle of incidence ofthe reflected light from the detector means and the information from thestoring means and for determining a category of gas volume of themultiphase fluid in the containing means according to the data relatedto the angle of incidence of the reflected light using the Brewsterangle of gas and the critical angle of gas.
 2. The apparatus of claim 1,wherein the processing means is also for deriving curvature informationof the fluid for use in determining a category of gas volume of thefluid.
 3. The apparatus of claim 2, wherein the processing means is alsofor determining baseline information of the fluid for use in determininga category of gas volume of the fluid.
 4. The apparatus of claim 3,wherein the processing means is also for analyzing gas step height todetermine that the fluid has a gas volume within one of medium gas andrelatively high gas categories.
 5. The apparatus of claim 4, wherein theprocessing means is also for analyzing the curvature information and thebaseline information with the gas step height to determine that thefluid has a gas volume within one of relatively low gas and no gascategories.
 6. The apparatus of claim 5, wherein the detector meanscomprises a plurality of detector elements, each arranged to detectlight having a corresponding angle of incidence within a range thatextends from less then the Brewster angle to more than the criticalangle of gas.
 7. The apparatus of claim 6, wherein the source meanscomprises an infrared emitting diode.
 8. The apparatus of claim 7,wherein the optical means comprises a prism substantially transparent tothe light, the prism forming the interface with the fluid in thecontaining means.
 9. The apparatus of claim 8, comprising a p-polarizerbetween the infrared emitting diode and the prism.
 10. The apparatus ofclaim 9, comprising a focusing means for focusing the light from thep-polarizer onto the interface.